Majority of human prostate cancers overexpress Bcl-2 and/or Bcl-XL, the important negative regulators of apoptosis. Gossypol(-), a natural product from cottonseed, has recently been identified as a potent small molecule inhibitor of both Bcl-2 and Bcl-XL. In the current study, we tested our hypothesis that Gossypol(-) may improve prostate cancer’s response to radiation by inhibiting the anti-apoptosis activity of Bcl-2/XL and making cancer cells more sensitive to radiation therapy. Our data show that Gossypol(-) inhibits tumor cell growth and induces apoptosis in human prostate cancer PC-3 cells with high levels of Bcl-2/XL proteins, but has minimal effect on normal cells with low Bcl-2/XL. In clonogenic assays, treatment of PC-3 cells with Gossypol(-) significantly reduced radiation resistance of PC-3, resulted in 10- and 20-fold reduction of colony formation at 8 Gy. Fluorescence resonance energy transfer (FRET) assay using Bcl-XL-CFP and Bax-YFP or Bad-YFP co-transfected cells suggests that Gossypol(-) potently blocks the interaction of Bcl-xl with Bax and Bad in live cells, in a time- and dose-dependent manner, supporting that Gossypol(-) induces apoptosis, at least in part, through inhibition of the anti-apoptotic protein Bcl-XL. Our in vivo studies using PC-3 xenograft models in nude mice show that orally administered Gossypol(-) has limited anti-tumor activity but achieves a much greater efficacy with tumor regression when used in combination with X-ray irradiation. Combination therapy of Gossypol(-) 10 mg/kg, p.o. q.d.5 x 4 weeks, with fractionated X-ray irradiation, 2 Gy q.d.5 x 3 weeks, achieved 96% tumor growth inhibition (T/C = 3.4%), significantly more effective than either Gossypol(-) or radiation alone (T/C = 96% and 37%, respectively) (p<0.01, n = 16). Similar results were observed with PC-3 tumors at size of 200 and 400 mm3 at the start of radiation, where only combination therapy resulted in tumor regression. For PC-3 tumors with starting size of 200 mm3, Gossypol(-) plus radiation achieved significant tumor growth delay (T-C = 54.5 days) as compared with Gossypol(-) or radiation alone (T-C = 0 and 8.5 days, respectively). In situ TUNEL-staining showed significantly more apoptotic cells induced in the tumors treated with Gossypol(-) plus radiation than either treatment alone. Anti-CD31 immunohistochemical staining indicates that Gossypol(-) plus radiation significantly inhibited the tumor angiogenesis. No significant toxicities were observed. Our results demonstrate that Gossypol(-) significantly enhances the anti-tumor activity of radiation therapy in vitro and in vivo, and may represent a promising new anticancer agent with a novel molecular mechanism. Molecular targeted therapy with Gossypol(-) may improve the outcome of current radiation therapy for human prostate cancer with Bcl-2/XL overexpression.

[Proc Amer Assoc Cancer Res, Volume 45, 2004]